This invention relates to a method of dry offset non-overlap halftone-dot printing of a realistic design with light and shade on the outer surface of cylindrical containers such as a two-piece can (used for accommodating beer, mineral drinks, juices, non-juice drinks and sport drinks and consisting of a can body with an intergral bottom and a lid).
The technique of non-overlap halftone-dot printing on the outer surface of cylindrical containers such as can bodies using a dry off-set printer capable of high speed printing is well known in the art (as disclosed, for instance, in Japanese Patent Laid-open Publication No. 162241/1988).
The Japanese Patent Laid-open Publication No. 162241/ 1988 has disclosed a printing method comprising the steps of color separation of a colored original of realistic design into seven colors, i.e., cyan, magenta, yellow (these three colors being herein after sometimes referred to as primary colors), blended cyan-magenta, blended magenta-yellow, blended yellow-cyan (these three colors being hereinafter sometimes referred to as secondary colors) and black (this color being hereinafter sometimes referred to as tertiary color), producing plates having raised portions corresponding to contracted images of the individual colors obtained by the color separation, for each color, supplying inks of seven different colors, kneaded with a number of ink kneading rollers on to the raised portions of the individual plates set on a plurality of plate cylinders, transferring the inks on the individual raised portions onto a common blanket provided on a transfer roller, and further transferring all the inks from the blanket simultaneously onto a cylindrical can body supported on a rotor.
The term "cyan", "magenta" and "yellow" as used herein may respectively include any resembling colors thereof. For example, cyan means cyan and any color resembling cyan.
For producing the plates used in the aforementioned printing method, the color components of the colored original are first separated into four colors, i.e., three primary colors of cyan, magenta and yellow, and black. Then, according to a conventional method the change to half tone is effected by screening the respective colors at different angles. Subsequently, those portions of dot areas of halftone images of cyan and magenta that overlap with each other to give a resultant optical image of a secondary color of cyan-magenta in the colored original are changed to corresponding areas of blended cyan-magenta, those portions of dot areas of halftone images of cyan and yellow that overlap with each other to give a resultant optical image of a secondary color of cyan-yellow are changed to corresponding areas of a blended cyan-yellow, those portions of dot areas of halftone images of magenta and yellow that overlap with each other to give an optical image of a secondary color of magenta-yellow in the colored original are changed to corresponding areas of blended magenta-yellow and those portions of dot areas of halftone images of cyan, magenta and yellow that overlap all together to give an optical image of a tertiary color in the colored original are changed to corresponding areas of tertiary color. Further, those portions of dot areas of halftone images of cyan, magenta, yellow and black that overlap all together to give an optical image of tertiary color in the color original are changed to corresponding areas of tertiary color. In this way, the color components of the colored original are changed to seven color elements. Thereafter, the individual halftone-dot images are contracted, and then plates having raised portions corresponding to the individual contracted images are produced.
In this printing method, in addition to inks of the basic colors, i.e., cyan, magenta, yellow and black, inks of intermediate colors, i.e., cyan-magenta, magenta-yellow and yellow-cyan, are used. Thus, it is possible to obtain a print of a realistic colored original with light and shade by using a non-overlap printing method.
However, in the above prior art method the change to halftone for the four basic separated colors obtained from the colored original is done in the conventional way of screening at different respective angles for the four basic colors. Therefore, when the halftone-dot images of three colors, i.e., cyan, magenta and yellow, obtained after change-to-halftone, are overlapped to produce a compounded image reproducing the image of the colored original, individual dots of respective colors are arranged to produce a pixel to form a compounded image reproducing the image of colored original as shown in FIG. 10(1), for instance. By changing the area where cyan and magenta overlap each other to an area of blended cyan-magenta CM, the area where magenta and yellow overlap each other to magenta-yellow MY, the area where yellow and cyan overlap each other to yellow-cyan YC and the area where the three colors overlap all together to tertiary color B, resultant color elements are arranged as shown in FIG. 10(2). In this case, the shape and sizes of the individual dots for respective colors are as shown in FIG. 10(3).
As is shown, since a pixel representing an intermediate color according to the prior art method is expressed by a combination of at most seven colors, the individual plates essentially have to have an extremely large number of small dots as shown in FIG. 10.
When such plates are used for dry off-set printing, a phenomenon called "dot gain" takes place, i.e., dots and lines are enlarged on the printed surface. This may make highlight areas of the print darker or destroy an original color balance. If image reduction is effected in advance of producing the plates to make up for the dot gain at the time of the off-set printing, small dots may be reduced to such extent that they do not appear on the plates. When such on plates are used for printing, highlight areas of the print may not be reproduced. Thus, extreme deterioration of the image quality of the print is inevitable.
U.S. Pat. No. 4,519,310 discloses a technique of halftone-dot printing on the outer surface of a cylindrical container such as a can body using a dry off-set printer. According to this technique of halftone-dot printing (of either non-overlap or overlap type) on the cylindrical container outer surface, inks kneaded by a plurality of ink rollers are transferred onto the raised portions of plates set on a plurality of plate cylinders and then onto each blanket provided on a transfer roller, the ink on each blanket is transferred onto a can body supported on each can support of a rotor, one color per a revolution of the can body, and the rotation of the transfer roller is synchronized with the rotation of the rotor.
With this printing technique, mixing of individual inks is prevented by setting different tack values for the respective inks such that it is the ink with the highest tack value which is first transferred onto the can body and the tack values of the respective inks become gradually lower in the order of transfer onto the can body.
This printing technique, however, has the following disadvantages.
(1) Even where only the basic four colors are used in printing, the can body has to be rotated four rotations so that the printing speed is reduced to at least one-fourth of that of a conventional printer.
(2) The can support for supporting the can body requires a complicated mechanism for reliably holding the can body during printing and a mechanism for providing high accuracy of synchronized rotation to avoid off-registration of the respective colors. Therefore, the printer must have extremely high accuracy.
(3) Where an ink overlaps with another ink which has not been baked dry, it is necessary to provide the inks with different tack values in order to prevent reverse trapping of an ink (i.e., a phenomenon of pulling an ink transferred onto the can body by another ink), and control and maintenance of such a tack difference is complicated and difficult.
This technique, therefore, can not solve the problems which the present invention seeks to solve.
Japanese Patent Laid-open Publication No. 126750/1988 discloses a different printing method. In this method, a color design to be provided on the outer surface of a cylindrical container is color-separated into color components of like color hue which are not affected by mixing of inks, and inks of the like color hue obtained by the color-separation are prepared. Each plate is formed for an image of each like color hue. Each ink kneaded by a number of ink rollers is supplied to raised portions of each plate set on a plurality of plate cylinders and transferred onto a common blanket provided on a transfer roller. Inks of all colors on the blanket are simultaneously transferred onto the cylindrical container supported on the rotor. The rotation of the transfer roller is timed with that of the rotor to effect continuous printing by a dry off-set printing.
This method has the following disadvantages.
(1) Since the color design is color-separated into like color hues, sophisticated color balance control is required, and the color separation is complicated and difficult.
(2) The color-separation of a color design into like color hues means that a color design consisting of a plurality of different color hues have to be separated into a larger number of colors. Therefore, where the available number of colors is limited, a desired color design may not be used for printing. The application of this printing method, therefore, is limited to color designs consisting of like color hues.
Japanese Patent Laid-open Publication No. 162240/1988 also discloses a method of printing and a method of producing plates used for the same printing as disclosed in Japanese Patent Laid-open Publication No. 162241/1988. With these methods, a color design to be provided on the outer surface of a cylindrical container is color-separated into seven different colors, i.e., cyan, magenta, yellow, cyan-magenta, magenta-yellow, yellow-cyan and black, plates with raised portions corresponding to contracted images of the separated colors are produced for each color, seven different color inks kneaded by many ink kneading rollers are supplied to the raised portions of the plates set on a plurality of plate cylinders and are transferred onto a common blanket provided on a transfer roller, and the inks of all colors on the blanket are simultaneously transferred onto a cylindrical can body supported by a mandrel on a rotor.
With these methods, however, one intermediate color pixel of the color design to be printed on the cylindrical container outer surface is expressed by a combination of at most seven different colors. Therefore, the individual plates essentially have to have extremely larger numbers of small dots. The use of such plates for dry off-set printing leads to a phenomenon called "dot gain", i.e., the size increase of dots and lines, resulting in the size increase of dots on the surface to be printed. Therefore, highlight areas may become darker, or the color balance may be destroyed. If an image reduction is made in producing the plates to make up for the dot gain, small dots may be reduced to such extent that they disappear from the plates. With such plates, reproduction of a highlight area can not be obtained, or at any rate, the image quality of a print is extremely deteriorated.